1. Field of the Invention
The invention relates to an electric power steering system that assists a steering operation by transmitting driving force generated by an electric motor to a steered mechanism of a vehicle.
2. Description of the Related Art
There is an electric power steering system that transmits driving force generated by an electric motor to a steered mechanism of a vehicle, thereby assisting a driver's steering operation. The electric motor is controlled by an electronic control unit on the basis of, for example, a steering torque applied to a steering wheel and a vehicle speed. Specifically, detection signals from a steering torque sensor that detects a steering torque and a vehicle speed sensor that detects a vehicle speed are input into the electronic control unit. The electronic control unit sets a target current value on the basis of the input signals from the sensors and executes feedback control on the electric motor on the basis of the target current value.
When a tire bursts, the steering torque varies. The variations in the steering torque hinder a steering operation and thus should be suppressed. Techniques for suppressing vibrations due to disturbance through electric control are described in Japanese Patent Application Publications No. 2003-2215 (JP 2003-2215 A), No. 2007-112189 (JP 2007-112189 A), and No. 2005-112044 (JP 2005-112044 A). However, the variations in steering torque when a tire bursts are considerably large, and it is thus difficult to suppress such large variations in the steering torque through the electrical control. Therefore, a mechanical vibration suppressing mechanism such as a bushing is required in the related art.